Hi Wamuu,
I am pretty sure the mobs that had a summoned minion/pet would despawn in vanilla after killing the summoner however I have noticed they work in a strange way on elysium.
If you pull the mob without pulling it's minion/pet (which is possible) and kill it, the minion/pet will despawn.
If you kill the summoner while having their summoned pet/minion in combat it will not despawn,
Brude